In conventional bump cams, a driver is pushed downwardly by an upper die into the bump cam housing. The driver has an inclined lower end which engages a complimentary inclined end of a tool holder. As the driver moves downwardly, the inclined surfaces cause a tool carried by the tool holder to be shifted outwardly to punch a hole, for example, in the sheet metal workpiece. After a period of time, the laterally directed loading imposed on the tool holder by the inclined face of the driver causes a wearing of the bearing surfaces between the tool holder and the bump cam housing, and the tool carried by the tool holder will drift off center so that the hole being punched will not be in the proper location.
In addition, the construction of conventional bump cams causes a high unit loading at the bearing surfaces within the bump cam that leads to rapid wear and shorter cam life.